1. Field of the Invention
The present invention relates to a fluid dynamic bearing device for supporting a shaft member with a dynamic pressure of a lubricating oil in radial bearing gaps between an inner peripheral surface of a bearing member and an outer peripheral surface of the shaft member in a manner that relative rotation of the shaft member is freely allowed, and to a bearing member used in the fluid dynamic bearing device.
Fluid dynamic bearing devices have high rotational accuracy and are excellent in quietness, and hence are suitably used in the following: a spindle motor for various disk drive devices (such as a magnetic disk drive device for HDDs and an optical disk drive device for CD-ROMs and the like); a polygon scanner motor for laser beam printers (LBPs); a color-wheel motor for projectors; or small motors such as a fan motor used, for example, for cooling electrical devices.
For example, in the fluid dynamic bearing device disclosed in Patent Literature 1, radial bearing surfaces (regions in which dynamic pressure generating grooves are formed) are formed at two regions spaced apart from each other in an axial direction of an inner peripheral surface of a bearing member, and a thrust bearing surface (region in which dynamic pressure generating grooves are formed) is formed in an end surface of the bearing member. A middle relief portion slightly larger in diameter than the radial bearing surfaces at the two regions is provided therebetween in the axial direction. With this, rotational torque of the shaft member is reduced.
As a method of manufacturing the bearing member provided with the middle relief portion as described above, for example, as disclosed in Patent Literature 2, there is a method of forming dynamic pressure generating grooves in an inner peripheral surface of a bearing member, including: compressing, under a state in which a core rod is inserted along an inner periphery of the bearing member, parts of an axial region of an outer peripheral surface of the bearing member; pressing the inner peripheral surface of the bearing member onto molding dies formed on an outer peripheral surface of the core rod; and transferring shapes of the molding dies to the inner peripheral surface of the bearing member. In this case, a region (middle relief portion) out of regions in which the dynamic pressure generating grooves are formed (radial bearing surfaces) is formed to be larger in diameter than the radial bearing surfaces.
2. Description of Related Art
Patent Literature 1: JP 2007-46665 A
Patent Literature 2: JP 11-037156 A